1. Field of the Invention
The present invention relates to a backlight device and a liquid crystal display device using the backlight device, and more specifically to a backlight device of a so-called side light type using a light guide plate and to a liquid crystal display device using the backlight device.
2. Description of the Related Art
In a non-self-emitting display device such as a liquid crystal display device, a so-called backlight device is typically arranged which irradiates a display panel from behind. This backlight device is of a direct light type, side light type, planar light source type, or the like. For a small-sized display device, in terms of slimming-down and weight saving, those of the side light type are currently widely used.
The backlight of this side light type irradiates a back surface of the display panel by making light from a light source incident on a side surface of a light guide plate and exit as a flat light source from a front surface thereof by making the light travel in the light guide plate in a manner so as to be totally reflected therein while making part of the light reflected by a reflective sheet fitted on a back surface of the light guide plate or by changing the propagation direction of the light with a medium border surface and a pattern formed on the light guide plate.
Displacement of the light guide plate built in the backlight device of the side light type due to vibration, impact, and the like influences the display quality. Moreover, the displacement may cause a collision between a member, such as a light source lamp, surrounding the light guide plate and the light guide plate, resulting in damage to the surrounding member and the light guide plate itself.
Thus, various technologies for fixing a light guide plate to prevent its displacement have been suggested. For example, JP-A-H6-308489 suggests a technology of fixing a light guide plate in a case by providing a depression or a projection on a surface other than a light incident surface of the light guide plate, also providing a projection or a depression at a position of the case facing this, and engaging the projection and the depression with each other. In addition, JP-A-H 10-48427 suggests a technology of positioning and fixing a light guide plate by providing a stopper on a light exit surface side at the leading end facing a light incident surface of the light guide plate or a surface side facing the light exit surface side.
As shown in FIG. 8, forming projections 114a and 114b on side surfaces of a light guide plate 11′, forming engaging grooves 132a and 132b in a case 13, then fitting the light guide plate 11′ to the case 13 with the projections 114a and 114b of the light guide plate 11′ being engaged with the engaging grooves 132a and 132b of the case 13 prevents displacement of the light guide plate 11′ caused by vibration and impact. However, since the projections 114a and 114b project abruptly from the light guide plate 11′, light reflects and diffuses at boundaries between the light guide plate 11′ and the projections 114a and 114b, which disturbs the light propagation uniformity. This results in a problem that a bright line and a dark line occur near the roots of the projections 114a and 114b, causing non-uniformity in the brightness of illumination light and further decreasing the light emission efficiency. Although the projections are formed on the side surface of the light guide plate 11′ of FIG. 8, brightness non-uniformity is similarly observed in a case where projections are formed on the back side of the light guide plate 11′.